1. Field of the Invention
The present invention relates to a semiconductor memory device which rewritably stores data in a plurality of memory cells formed at intersections of a plurality of word lines and a plurality of bit lines, and particularly relates to a semiconductor memory device employing an embedded bit line structure in which a bit line is arranged below a vertical transistor formed at each memory cell.
2. Description of Related Art
In order to reduce the chip area of a semiconductor memory device such as SRAM or DRAM, it is important to reduce an occupied area of a peripheral circuit disposed adjacent to a memory cell array, as well as to reduce an area of the memory cell array itself. Therefore, various layout methods for reducing the occupied area of the peripheral circuit such as sense amplifiers in the semiconductor memory device have been proposed (see Patent References 1 to 4). Also, in connection with these layout methods, methods have been proposed in which transistors regularly arranged in the memory cell array are used as constituent elements of other circuits whose purpose is different from that of each memory cell,
The Patent Reference 1 discloses a technique for improving connection reliability in a large capacity DRAM having memory cells of a COB (Capacitor Over Bit Line) structure without providing a buffering area for a step between the memory cell array and the peripheral circuit. According to the technique in the Patent Reference 1, a method for employed in which MOS transistors included in the peripheral circuit are formed with the same layout and the same structure as a select transistor of the memory cell.
The Patent Reference 2 discloses a semiconductor memory device capable of reducing the area in an arrangement with small sense amplifiers between a plurality of memory cell arrays of SRAM without providing dummy memory cells. According to the technique in the Patent Reference 2, the small sense amplifiers can be configured by directly utilizing the transistor arrangement of the memory cells.
The Patent Reference 3 discloses a semiconductor memory device in which a capacitor of a memory cell of DRAM is used as a decoupling capacitor for a power supply wiring so as to reduce an area where other decoupling capacitors are formed. According to the technique in the Patent Reference 3, memory cells existing in a part of the memory cell array are connected in parallel, and the decoupling capacitor is achieved by controlling a select transistor to be constantly on.
The Patent Reference 4 discloses a technique of providing dummy cells in the memory cell array and using them in a read operation. According to the technique in the Patent Reference 4, the dummy cells are formed using capacitors having the same structure as of a normal cell, and a layout for the normal cell can be utilized only by providing an additional write MOS transistor for controlling.    Patent Reference 1: Laid-open Japanese Patent Publication No. Hei7-122654    Patent Reference 2: Laid-open Japanese Patent Publication No. 2001-14861    Patent Reference 3: Laid-open Japanese Patent Publication No. 2003-332532    Patent Reference 4: Laid-open Japanese Patent Publication No. 2005-51044
In order to miniaturize the memory cell of the DRAM, it is desirable to form the memory cell for which a vertical transistor structure is employed as the select transistor. Generally, in the memory cell formed in this manner, an embedded bit line structure is employed in which a capacitor is formed above the vertical transistor and a bit line is arranged below the vertical transistor. Further, in the peripheral circuit of the DRAM, particularly an area occupied by sense amplifiers arranged adjacent to the memory cell array is dominant. However, on the premise of the memory cell array employing the vertical transistor structure and the peripheral circuit including a sense amplifier circuit and the like in the DRAM, it is difficult to apply any of the above-mentioned conventional techniques.
Since the technique disclosed in the Patent Reference 1 is assumed to be applicable to a memory cell using a planer type MOS transistor, it is not applicable to the vertical transistor structure. The technique disclosed in the Patent Reference 2 is applicable only to the memory cell array of SRAM, and thus is not applicable to the memory cell array of DRAM. The technique disclosed in the Patent Reference 3 is not applicable to a transistor included in the sense amplifier. The technique disclosed in the Patent Reference 4 is applicable only to the dummy cell, and is not applicable to a transistor included in the sense amplifier. In this manner, according to the above conventional techniques, a problem exists in that the purpose of reducing the chip area cannot be achieved by using the vertical transistor for the memory cell of DRAM and by applying it to the transistor included in the sense amplifier.